Method and apparatus for rapid prototyping of monolithic microwave integrated circuits

ABSTRACT

A system is provided for rapidly prototyping monolithic microwave integrated circuit modules to provide an experimental structure that can be physically made to operate by stereolithographically rendering an SLA part directly from the designer&#39;s drawings and by coating the part with a conductive layer, with the smoothness being better than 0.001 inch and with the dimensional accuracy being better than 3 mils for some applications. In one embodiment, the coating for the SLA model is made from either rhodium or gold having a thickness of less than 1/10,000 th  of an inch, with a nickel barrier being first deposited followed by the gold. The particular SLA plastic is a high temperature plastic to prevent bubbling during the metal deposition step which in one embodiment involves sputtering. The coated model has a microfinish of Ra&lt;0.001 inch, with active devices being electrically attached to the coating through the use of conductive adhesives or eutectic solder. This provides a device in which active devices are attached to the layer without soldering and without introduction of heat which might destroy the metallized layer and alter and the critical dimensions.

FIELD OF INVENTION

This invention relates to rapid prototyping of microwave modules andmore particularly to the utilization of metal coating of astereolithographic assembly (SLA) model for the provision of a physicalmodel that can function as an active microwave electronic circuitsuitable for testing and demonstration.

BACKGROUND OF THE INVENTION

Microwave and monolithic microwave modules have been formed in which thecircuits themselves are mounted into a passive electrically conductivehousing or carrier and have active electronic elements such as filters,mixers, switches, amplifiers, and the like epoxied or soldered to theinterior of the housing, with a top lid hermetically sealing thehousing. The devices are made of plates or sheets of conductive materialsuch as metals which have to be manufactured by ion milling toexceedingly high tolerances.

The reason for the high tolerances is that one seeks to minimize lossesthrough the monolithic microwave integrated circuit, with the lossesresulting from dimensional inaccuracies which can cause detuning of thecircuit because it varies the propagation structure and becauseroughened surfaces result in mismatches and/or circuit losses.

In order to provide a prototype for a final microwave assembly, theprototyping process in the past has required multiple different stepsstarting with the circuit designer. The circuit designer will design amodule utilizing some sort of drawing. The drawing is then transferredto a machine shop which requires that the drawing be put in a CAD CAMformat so that a solid blank of material can be hogged out and thenmicrofinished to the required dimensions by etching, grinding ormilling. This requires a numerical control (NC) tape, with the milledproduct then being transferred to a plating shop at which point, forinstance, gold plating is sputtered onto an appropriately patternedmetal housing.

Because of the multiple steps involved, oftentimes, the resultingprototype doesn't fit or is improperly dimensioned so that one then hasto go back through the above process in order to provide a prototypemodel that will operate properly.

It will be appreciated that in monolithic microwave integrated circuits,a number of active elements are soldered or conductive-epoxied to theplating in order to provide for the appropriate functioning of thecircuit.

While soldering per se is not a problem when the module housing is madeof solid metal, it does become a problem if other housing materials areutilized.

The above process may take a matter of several weeks in order to providea prototype which is testable and for this reason there is a need for arapid prototyping process which reduces costs and turnaround time tohours or days.

SUMMARY OF THE INVENTION

Rather than utilizing milling techniques for hogging out a billet ofmetal in order to provide the housing portion of the circuit, in thesubject invention, a stereolithographic rendering is made directly fromthe designer's drawings.

In one embodiment, the stereolithographic assembly (SLA) is provided bybuilding up or ablating a high temperature plastic, with the hightemperature plastic being required for a subsequent plating orsputtering process in which conductive metal is sputtered or plated ontothe plastic to provide the conductive surface for the housing.

In one embodiment, when utilizing gold, a nickel coating is firstdeposited followed by the sputtering of the gold, with the gold surfacesprovided with a microfine finish so that circuit losses are minimizedand so as to preserve dimensional accuracy. Moreover, rather thansoldering the active elements to the housing as was done in the past,conductive adhesive is utilized for electrically connecting and mountingthe active elements to the sputtered conductive layer.

The housing itself is made of a material which has a temperaturecoefficient of expansion such that upon heating in the metallizingprocess the originally designed dimensions as well as edges and cornersare maintained. The plastic in one embodiment is of a high temperaturematerial such that the sputtering temperatures utilized in thesputtering process do not result in bubbling of the metallized coatingwhen, for instance, metal flash techniques are utilized to provide thehousing with the appropriate layer. In one embodiment, the layer is lessthan or 1/10,000^(th) of an inch in thickness which while satisfactoryfor prototyping purposes is not necessarily desirable if the activecomponents are to be soldered to the housing. Moreover, in the subjectprocess the active components are mounted with conductive adhesive suchas a silver epoxy, thus eliminating the requirement for the applicationof heat.

Moreover, the prototype is in general designed for low temperatureapplications where heat sinks need not be provided.

The result is a non-metallic structure which is much lighter than thebrassboard prototyping utilized in the past, with the smoothness of thefinish for the housing walls and the metallization reducing lossesassociated with the propagation structure of the wave energy altered bydimensional and surface factors.

Thus the key to the subject invention is a microfine surface having aroughness (surface height deviation, Ra) less than 1/1000^(th) of aninch, along with the holding of tight tolerances of +/−3–5 mils throughthe utilization of high temperature plastics and currently available SLAprocesses. Note that Ra is specified by ASME in Standard B46.1-1995.

The above addresses the issue of the propagation structure of waveenergy which requires very smooth, very light, very tight tolerancestructures which when completed in a clam shell type structure bysnapping together the parts results in a structure that can bephysically made to operate. Thus signal reduction or loss is kept to aminimum due to the microfine surface structure which if rough can causedetuning or changes in the waveguide frequency cut-off. When themicrowave module is configured as a high pass filter, the frequencycut-off is controllable through the use of the subject techniqueespecially when, for instance, transformers are internally installed inthe microwave housing.

It is thus possible with the stereolithographic rendering and coatingtechniques to prepare a suitable prototype in a matter of days asopposed to a matter of weeks and to be able to physically try out anidea and validate that it works in a very short period of time and atlower cost.

In summary, a system is provided for rapidly prototyping monolithicmicrowave integrated circuit modules to provide an experimentalstructure that can be physically made to operate bystereolithographically rendering an SLA part directly from thedesigner's drawings and by coating the part with a conductive layer,with the smoothness being better than 0.001 inch and with thedimensional accuracy being better than 3 mils for some applications. Inone embodiment, the coating for the SLA model is made from eitherrhodium or gold having a thickness of less than 1/10,000^(th) of aninch, with a nickel barrier being first deposited followed by the gold.The particular SLA plastic is a high temperature plastic to preventbubbling during the metal deposition step which in one embodimentinvolves sputtering. The coated model has a microfinish of Ra<0.001inch, with active devices being electrically attached to the coatingthrough the use of conductive adhesives or eutectic solder. Thisprovides a device in which active devices are attached to the layerwithout soldering and without introduction of heat which might destroythe metallized layer and alter and the critical dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in connection with the Detailed Description in conjunctionwith the Drawings, of which:

FIG. 1 is an isometric view of a monolithic microwave integrated circuitin the form of a switch showing a metallized SLA model with componentsand/or a circuit board to be adhesively mounted in a housing;

FIG. 2 is a top view of the SLA part of FIG. 1 showing the placement ofthe active and passive parts therein;

FIG. 3 is a schematic diagram of the monolithic integrated circuit ofFIGS. 1 and 2; and,

FIG. 4 is a flow chart for the process of making the housing of FIG. 1to produce a finished prototype.

DETAILED DESCRIPTION

Prior to discussing the housing and the criticality of the dimensions,it will be appreciated that microwave integrated circuits havecomponents such as conductors, inductors, capacitors, resistors andactive circuits which must be positioned relative to the housing withtolerances in the range of thousandths of an inch. What this means isthat when one seeks to design a housing into which the components are tofit, there are some critical dimensions which become operative due tothe microwave frequencies.

Rather than simply locating components on a circuit board in anindiscriminate fashion, one needs to pay attention to various dimensionsbecause the wavelengths are so short. If the placement of a part orpositioning of a line or trace is inconsistent with good designpractice, the performance of the circuit can be degraded or can even beunacceptable.

For instance, misplacement of the components may cause detuning and maycause undesirable reflections, because the circuit size is on the sameorder as the wavelength of the energy it handles. Thus the placement ofthe parts becomes increasingly critical when one gets above 1,000megahertz or one gigahertz.

As an example, if the input pin to a microwave integrated circuit ismisaligned with the input trace on a printed circuit carried in thehousing, then one can have a very high VSWR. Moreover, for microwaveswitches, especially diode switches, if the diodes are improperlypositioned, instead of getting a switch action, one can get a splittingaction or a cross-coupling.

Further, it is extremely important to have a housing with a very smoothsurface to bond to so that one gets a uniform ground distributionsubstrate. If one does not have a uniform ground, one gets a non-uniformresponse from the circuit. Additionally, for the channels or walls ofthe structure, they must be co-planar in structure to within severalthousandths of an inch.

As a further consideration, it is important with all microwave circuitsto prevent unwanted modes from propagating within the housing. Forenergy propagating in an unintended mode, one can get resonances,reinforcements or cancellations. In summary, one has to satisfy boundaryconditions that establish the dimensions of the circuit.

Referring now to FIG. 1, in order to rapidly prototype a monolithicmicrowave integrated circuit 10, one forms an SLA model or housing 12coated with a conductive layer. The coating may be patterned to exist atleast along the inside wall 14 of model 12, along the bottom of thehousing and along a peripheral top lip 18 such that the coated wallsform a housing.

What is provided is a metallized non-conductive housing made with SLPtechniques. The coated housing has electrical properties which duplicatethat which would be expected from a brassboard prototype including ametallized hogged out and milled housing. Note that the activecomponents may be a transformer, an amplifier, circuit diodes, or maybe, for instance, an inductor used to give the monolithic microwaveintegrated circuit certain characteristics so as to provide a filter, anamplifier, a mixer, a circulator switches, or other microwave circuits.

For illustrative purposes, what is pictured in FIG. 1 is a monolithicmicrowave integrated circuit in the form of a microwave switch. It hasconnectors 22 serving as input and output connectors, with the outputbeing switched by a diode switching circuit 24 such that signals ontrace 26 are switched between trace 28 and trace 30.

As can be seen, an integrated circuit substrate or circuit board 32carries traces 26, 28 and 30, with diode switch circuit 24 also carriedby the integrated circuit board.

With all of the components, either passive or active, mounted on theintegrated circuit board, it is very important that the circuit boardfit tightly within the microwave cavity 34 formed by walls 14 andhousing 12. As can be seen, the integrated circuit board fits within thecavity so snugly that, for instance, an end 33 of trace 30 is directlyaligned with a pin 35, which extends into cavity 34 from the associatedconnector.

As mentioned hereinbefore, any misalignment either in the lateral orvertical direction of pin 35 with respect to end 33 of trace 30 cancause intolerable VSWR to occur.

Also, the placement of various active and passive elements, while beingmounted on the integrated circuit board 32, still must be placed atpredetermined distances from walls 14 of cavity 34. For instance,microwave integrated circuit chips 38, which carry numbers of active andpassive components, must have their positions at rigidly specifieddistances from wall 14 of housing 12 so that the circuit will operateproperly. Also the placement of capacitors 39 and inductors 40 for DCswitch biasing must likewise be precise.

Thus the microwave switch of FIG. 1 has a number of capacitors 39 andinductors 40, the proximity of which to walls 14 is critical to theproper operation of the circuit.

This being the case, it is very important to not only properly locatethe active and passive components on printed circuit board 32, but alsoone must be doubly sure that printed circuit board 32 is exactlypositioned with respect to the walls of cavity 34 and fit tightly withinthe cavity.

Referring to FIG. 2, a schematic top view of the microwave switch ofFIG. 1 is shown in which the various components shown in FIG. 1 arenoted as having like reference characters. Here it will be seen thatthere are critical dimensions for wall 14 as illustrated by double-endedarrows 41. In short, it can be seen that if printed circuit board 32 isnot precisely positioned within cavity 34, then various tight tolerancescannot be honored. For instance, as can be seen by double-ended arrow42, then unacceptable VSWR can occur if the input pin is not exactlypositioned relative to the trace to which it is to be connected.

It will also be appreciated that for a switch having a multiplicity ofinductors, capacitors and resistors along with active devices, the modeof propagation of the signals within the housing is critically dependentupon the accuracies to which the channelizing and other wall featurescan be formed.

FIG. 3 shows a simplified schematic drawing of the switch of FIG. 1,indicating the number of active and passive components which must beproperly positioned through location on a printed circuit board thatmust in turn be located precisely to the housing.

It is obviously important that one be able to provide rapid prototypingfor such microwave integrated circuits and to be able to quickly reshapethe housing when design errors are uncovered.

As is usual, the top of the housing may be sealed with a top plate (notshown) which may be hermetically sealed to top lip 18 of the housing.

There are several factors which contribute to the lossy microwavecircuits. The first is the smoothness of the metallization layer whichis to desireably have a microfine structure having a roughness less thanRa<0.001 inch.

Were the interior surfaces of the housing to have a roughness exceeding0.001 inch, then the wave propagation characteristics would be altered,in some cases resulting in detuning.

More importantly, if the dimensions of the housing are not exact, thendisplacement of the edges of wall 14 may cause detuning or may cause thehigh frequency cut-off of a high pass filter to change.

There are other structures within the housing which also are critical interms of dimensional accuracy. All of these dimensions are critical asto the wave propagation within the housing and any changes in theposition of these edges can result in detuning or changes in frequencyresponse.

What will therefore be appreciated is that while it is possible tocontrol the roughness of the interior walls of a brassboarded prototypeas well as critical dimensions, it is not immediately obvious howcritical surface smoothness and dimensional accuracies can be achievedwhen using SLA techniques.

Referring to FIG. 4, in order to form an SLA part as illustrated at 50one utilizes a specialized SLA material 52 which in a preferredembodiment is a high temperature plastic, the temperaturecharacteristics of which prevent bubbling of the subsequently appliedmetallization layer when the coating process is performed. Asillustrated at 54 the SLA part formed is coated with a layer ofconductive material 58 usually through sputtering which involves atemperature bath in excess of 1064° C. In one embodiment, the gold layeris preceded with a nickel metallization layer which is deposited at atemperature of 1455° C. While nickel and gold are deposited at theaforementioned processing temperatures, rhodium which can be substitutedis deposited at a temperature of 1964° C. The smoothness of theresultant layer is controlled at 56 in the formation of the SLA part.The resultant coated part has a suitable microfinish to which a circuitboard with active or passive components is adhesively mounted as shownin 60. Upon mounting of the board within the cavity of the SLA part andsealing with a conductive cover, one has a finished prototype asillustrated at 62.

It will be appreciated that the high temperature plastic which isutilized to form the SLA part must not deform or otherwise structurallybecome degraded at these deposition temperatures.

In terms of SLA part production, various standard SLA manufacturingprocesses are included within the scope of this invention. It isimportant that one control the dimensions and smoothness of the finishedpart and to do this, standard SLA manufacturing techniques may beemployed.

When a part having the appropriately configured housing dimensions andsmoothnesses is provided, conductive metallization is applied to thepart through sputtering, flash coating, or any other process which canprovide a microfinish having a finish roughness less than 0.001 inch.

Note that both active and passive components may be adhesively mountedin the housing so that with the addition of a hermetically sealingplate, a finished and testable prototype results. What will beappreciated is that one goes directly from the designer's drawings to anSLA part, completely bypassing the cutting of a metal blank either byion milling, preferential etching, grinding or other procedures.

As mentioned hereinbefore, the process of providing a brassboard typestructure is on the order of two weeks time, whereas providing an SLApart and flash coating it can be accomplished in a matter of days.

Thus what is provided is a prototype which is considerably less costlyto manufacture than the brassboard prototypes of the past, and can beformed in a rapid fashion to provide both testability anddemonstrability for the specific monolithic microwave integratedcircuit.

Having now described a few embodiments of the invention, and somemodifications and variations thereto, it should be apparent to thoseskilled in the art that the foregoing is merely illustrative and notlimiting, having been presented by the way of example only. Numerousmodifications and other embodiments are within the scope of one ofordinary skill in the art and are contemplated as falling within thescope of the invention as limited only by the appended claims andequivalents thereto.

1. A method for rapidly prototyping an active monolithic microwaveintegrated circuit, comprising the steps of: forming a housing usingstereolithographic techniques; providing a metallization layer onselected surfaces of the housing such that the selected metallizedsurfaces provide walls having a roughness, Ra, less than 0.001 inch;and, mounting a device on the metallized surface.
 2. The method of claim1, wherein the device includes an active device.
 3. The method of claim1, wherein the housing is made of plastic.
 4. The method of claim 3,wherein the plastic is a high temperature plastic.
 5. The method ofclaim 4, wherein the step of providing a metallization layer includesraising the metal to its melting temperature, and wherein the hightemperature plastic can withstand the melting temperature associatedwith providing the metallization layer.
 6. The method of claim 1,wherein the metallization layer includes gold.
 7. The method of claim 6,wherein the thickness of the layer is less than 1/10,000^(th) of aninch.
 8. The method of claim 6, wherein the gold metallization layer ispreceded with a layer of nickel.
 9. The method of claim 1, wherein theinitialization layer includes rhodium.
 10. The method of claim 1,wherein the mounting step includes a conductive adhesive.
 11. The methodof claim 10, wherein the conductive adhesive includes a silver paste.12. The method of claim 2, wherein the active device includes anamplifier.
 13. The method of claim 2, wherein the active device includesa filter.
 14. The method of claim 2, wherein the active device includesan inductor.
 15. The method of claim 2, wherein the active deviceincludes a capacitor.
 16. The method of claim 2, wherein the activedevice includes a switch.